NPL has been researching into wear resistance testing methods for some years, and have developed a good understanding of some of the important factors in test equipment design, as well as in wear mechanisms that can occur in ceramics and hardmetals. We have contributed to several VAMAS TWA1 interlaboratory exercises on this topic, and to the development of some of the first internationally accepted testing standards.
For hardmetal materials, the baseline test is ASTM B611, which is designed for mining/drilling grades of material. It is a particularly severe test involving rotating a steel wheel loaded with brown alumina grit against a flat test-piece. Remarkably, this test produces consistent results which enable a tight correlation between wear resistance and hardness to be established. However, this test is considered to be too severe for many applications which do not involve such a hard, coarse abrasive. A more generalised test has been proposed in ISO, and NPL is working to evaluate this test in comparison with ASTM B611 and variants of ASTM G65, the dry sand/rubber wheel test.
We are also evaluating erosion testing, and studying the mechanisms of erosive wear. In particular, we are using step-wise erosion by very small quantities of erodant in order to evaluate the sequence of surface damage and break-out in given microstructures. In hardmetals, for example, we have found that preferential removal of the binder phase leads to undermining of grains, and this is an important wear mechanism in fine grained materials (~1 ?m grain size), while grain fracture and chipping is more important in coarser grained materials (~20 ?m grain size).
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